UNTIL NOW, it was possible to hope that the damage caused by President Trump’s terrible incompetence, ignorance and impulsivity in foreign policy was largely theoretical, and possibly reparable. That is no longer true. The cost of his latest Syria blunder is unfolding before our eyes: Innocent lives lost. U.S. servicemen and -women betrayed. Butchering dictators emboldened. Dangerous terrorists set free. A ghastly scene is playing out, and it almost surely will get worse.

How often have Mr. Trump and his Republican enablers in Congress berated President Barack Obama for allowing Syria to cross his “red line” without dire consequences? None of them is entitled ever to mention that again.

Mr. Trump — with no consideration, no warning, no consultation with allies, no regard for the other nations that have fought alongside the United States and risked their men and women in the fight — has turned tail. In the past two years, courageous U.S. troops cooperated with our Kurdish allies to defeat the deadly Islamic State caliphate. These allies lost more than 11,000 men and women killed; the United States, a dozen. It was a rare U.S. success in the Middle East.

President Trump spoke Oct. 12 about his decision to withdraw U.S. forces from northern Syria, saying "let [the Kurds] have their borders." (The Washington Post)

The president has thrown it all away. His surrender is so hasty that U.S. forces could not execute a long-standing plan to take dozens of high-profile Islamic State detainees with them; we can expect to hear from those terrorists before long, in the region, in Europe or in the United States. The Islamic State is likely to exert its malign force again. The allies who fought alongside us are being slaughtered, and noncombatant women and children, too. Iran is strengthened, which threatens Israel. The murderous Syrian dictator Bashar al-Assad is strengthened, too. Russia is taking charge. America’s adversaries could not have scripted a better outcome.

Mr. Trump likes to preen and posture as a champion of American fighters. But what more bitter medicine could any commander in chief administer to U.S. troops than ordering them to abandon the comrades who fought alongside them? He likes to preen, too, as a great enemy of Iran, and even as he runs from Syria he is ordering 1,800 U.S. troops to Saudi Arabia, ostensibly to deter Iran. But that deployment, while proving the utter incoherence of his claim of “ending wars in the Middle East,” will have far less effect on Iran than the U.S. pullout from Syria, which opens the door for it to swell its influence there, on Israel’s border.

And speaking of preening: Republican senators such as Lindsey O. Graham (S.C.) and Marco Rubio (Fla.), who have cheered and celebrated Mr. Trump and his national security team, now huff and puff about imposing sanctions on Turkey as punishment for its invasion. Mr. Trump said Monday that he now supports such sanctions. But only one week ago he greenlighted Turkey’s incursion, and on Sunday he further encouraged it with his announcement of a hasty U.S. withdrawal. Turkish President Recep Tayyip Erdogan bears responsibility for Turkey’s depredations, of course. But if there is any coherence — or morality — in Mr. Trump’s position, he is doing a good job of concealing it.

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